Recirculating ink systems for inkjet printing, in which ink is recirculated through inkjet printheads, have benefits for the condition of the ink, particularly inks containing suspended solids, such as pigmented inks, reducing the likelihood of suspended solids settling out and also reducing air bubble formation, leading to improved printing performance and reliability.
Recirculating ink systems are not currently widespread, and some existing inkjet systems only allow for ink recirculation during non-printing periods, as the method employed for recirculation means that it is not possible to maintain the required constant negative pressure at the printhead nozzles whilst recirculation occurs.
Designs for ink systems to date that will allow recirculation to occur whilst maintaining a constant negative pressure, and thus allowing recirculation whilst printing, typically contain multiple printhead reservoirs, multiple pumps and multiple tubes to and from the main ink supply. See, e.g. U.S. Pat. No. 7,040,745. Multiple printhead reservoirs add cost and complexity to the ink system, as well as additional weight to the printhead carriage in a printing system. Multiple pumps add additional cost to the system, require control means and also are additional components that require service and replacement. Furthermore, it is desirable to minimise the number of tubes running from the main bulk ink supply to the printhead reservoir, as the reservoir is often mounted on a movable printhead carriage and thus the tubes must run through a flexible energy chain to the carriage. As the number of tubes used increases, the energy chain becomes larger, less flexible and more difficult to control. Known recirculating ink systems also generally have a relatively large ink capacity, e.g. at least about 1 liter.
WO 02/096654 (3M) discloses a recirculation system for pigmented inks using two reservoirs and a siphon effect to cause flow of ink through a printhead.
WO2006/030235 (Xaar) discloses in FIG. 8 an ink recirculation system using two chambers with weirs, with the chambers being maintained at different pressures to produce a pressure differential to drive ink through a printhead. Ink can flow from an inlet tank to an outlet tank, with the flow regulated by a valve in response to level sensors.
WO 2006/064036 (Agfa) concerns an ink circulation system for inkjet printing, with FIG. 3 showing apparatus including a printhead subtank with a weir separating first and second compartments of the subtank to produce a pressure differential for circulating ink through a printhead. The level of ink in the second compartment is controlled by level sensors acting to regulate a valve in flow lines leading to the main ink supply.